1. Field of the Invention
The present invention relates to optical fibers and, more particularly, to a single-mode optical fiber having an inter-core coated film of lithium-niobate fiber acting as a modulator/sensor.
2. Description of the Related Art
As shown in FIG. 1, an ordinary optical or plain fiber 10 is a wave-guide through which light is propagated by continuous total internal reflection within a glass central core 14 of the fiber. The glass surrounding the central core 14 is known as the cladding 18. Both core 14 and cladding 18 are dielectric materials. Because the index of refraction (n1) of core 14 is made to be higher than that of the surrounding cladding (n2), light waves remain trapped within core 14 during transmission. Depending on the wave modes that are utilized in transmission through the core 14, optical fibers 10 may be generally classified as single-mode or multi-mode fibers. Multi-mode fibers typically have core diameters of either 50 or 62.5 μm and can be used for sensors that reply on intensity modulation. Single-mode fibers, on the other hand, have core diameters typically in the range of 3 to 10 μm. Long distance communication and data transmission lines are generally comprised of single-mode sized fibers.
Advancements in photonics and fiber optic technology have revolutionized high speed data communication. In the current state of these technologies, the dominant role of optic fibers is for data transmission. Data, in the form of photons or light, is transmitted through optic fiber lines reliably and securely at very high speed and over a wide frequency band. To meet optimal requirements on integrity and quality of data transmission, the characteristics of the input and output light through the carrier optic fiber are desired to remain unchanged. Thus, the fiber optic line is ideally intended to function as a conduit preserving the intensity, frequency, phase and polarity of the light beam as best possible. In addition to high speed and broadband, fiber optic lines remain immune to effects from electromagnetic interference, interception, heating and arching. These advantages over wire and wireless data transmission media inspired aggressive investment in fiber optic trunk lines in the past decades.
The next phase of the fiber optic revolution is modulation while in transmission. Characteristics of light beam are desired to alter in proportion to physical, thermal, chemical and biological changes. The intention to maximize modulation of a selected light beam property or properties, however, is diametrically opposite to the desired objective of minimizing differences between input and output light in data transmission. Fiber segments that accentuate changes of characteristics desired to be preserved in data transmission are inserted in a network to detect or register relative changes. These fiber segments modulate transmitted light properties by virtue of induced changes in intrinsic or extrinsic conditions. Changes in intensity, frequency, phase, polarity and travel time have been exploited to develop a wide variety of sensors. The speed, reliability, accuracy, range and physical size make fiber optic sensors potentially attractive for a broad spectrum of applications. Incorporation of single or distributed fiber optic sensors within control loop circuits can lead to advances in smart materials and structures, intelligent transportation systems, energy conservation, clean environment, enhanced surveillance and in many fields of science and technology.
The main components of a control loop consist of transmission, detection, processing and actuation. Because of cost and relative development, practical circuits at present consist of mixed electronic and photonic components. The extent of photonic transmission would depend whether the circuit constitutes remote or local control and detection. For remote conditions, the bulk of the transmission can be in existing fiber optic lines. Local sensing and control loops can be fully electronic except for the sensing segment that would remain in photonic mode. Processing and actuating components would be in electronic mode for either local or remote loops. Circuits consisting entirely of photonic transmission, sensing, processing and actuation are not practical or economical at this time. The light input and output to and from the sensor can originate and terminate from a remote site or from close proximity to the sensor. The output is converted to electrical signal for processing and control decision. In turn, the processor controls actuators to initiate an adaptive response. This cycle continues to maintain set objectives for the system operation. An example would be to use sensor input and output to determine the axle load and wheelbase of a vehicle and the control decision may be to collect an appropriate toll charge for the vehicle. If successive sensors are linked, the toll can reflect also the travel time and speed of the vehicle. In addition, the toll can also reflect the time of day and level of traffic. Such a system would revolutionize the efficiency, energy conservation, law enforcement and safety of the highway system. While the technology for such a system exists, associated costs and complexity remain high.
With the above background provided, it is well known that splicing of very different core sized fiber segments introduces excessive leakage and thus single mode and multi mode fibers cannot be coupled. Apart from other inherent limitations, multi mode fiber sensors cannot be integrated within an existing trunk line communication fiber grid network. Commonly used fiber optic sensors for pressure and strain detection typically consist of segments of single mode sized fibers that contain Bragg interference gratings. When the sensor segment is subjected to changes in pressure and compliant strain, the grate spacing becomes altered. Such changes modulate the light wave passing through the sensor segment. A monochromatic light source of high intensity and interferometer detectors are usually required for this sensor system wherein the cost of such systems can be of the order of several thousand dollars.
U.S. Pat. No. 6,072,930 issued to Kornreich et al., the disclosure of which is hereby incorporated by reference in its entirety, discloses a method for placing a thin film of semi-conductor material between the core and cladding of a fiber preform, and drawing an optical fiber from the preform.
In the field of telecommunications, optical modulators in the form of thin films are widely used. Lithium niobate is a popular electro-optic material that is frequently used as a channel waveguide modulator with thin film metal electrodes being deposited on its planar surfaces. A prior art electro-optical modulator 38 is depicted herein at FIG. 2. As illustrated, in the lumped Mach Zehnder interferometer configuration waveguide 40, a lithium niobate coated rectangular wafer 44 is coupled into a fiber network. Light enters the network and is divided by a splitter 46 along two paths as shown; an upper path 48 containing the lithium niobate waveguide and a lower path 52 comprising an optical fiber with a known fixed time delay. The fiber path 48 containing the lithium niobate waveguide functions as a variable delay path due to the variation in the index of refraction of the lithium niobate structure when an electric field is applied. Light from the two paths 48, 52 is combined at the output end of the system and will constructively or destructively interfere depending upon the delay that is provided by the lithium niobate waveguide. A noted shortcoming of such a commercially available electro-optic modulator arises from the inefficient coupling of the rectangular waveguide structure into the otherwise all-fiber network. Coupling losses due to connection pig-tailing can approach or exceed fifty percent (50%). In addition, the particular way in which the lithium niobate crystals are grown may result in performance issues with such a device.
In the field of sensing, optical fibers wrapped around piezoelectric sensitive mandrels are often used to sense sound waves in underwater applications, such as oil and geological surveys, military surveillance, among others. As illustrated in FIG. 3, a plain optical fiber 60 is wrapped around a mandrel hydrophone 64, which is sensitive to sound, and also similarly wrapped around a mandrel 66 that is insensitive to sound. The wrappings are made in a Mach Zehnder interferometer configuration as described above; i.e., an input signal is split and directed along two different paths, one path 68 of which is sensitive to sound and the other path 69 which is not sensitive to sound. In the presence of sound vibrations, the sensitive mandrel 64 will sufficiently change to cause a slight path length change in the wrapped fiber 60, thereby effecting the phase of the signal in that leg of the device. When the signals recombine at the output, they will interfere either constructively or destructively. Again, integration losses for this type of device geometry are high.